Conventional detection mechanisms for detecting the bending angle of a metal sheet, provided in metal sheet bending machines are broadly classified into two major categories, namely, contact and non-contact.
An example of a detection mechanism of the contact type is set out in Japanese Patent Publication Laid-Open No. 1-273618 (1989). The above disclosure teaches a continuous follow-up angle detection device to which a rectangular link is applied and wherein the inclination of a probe in contact with the inclined surface of a metal sheet is read by an encoder provided within the link mechanism, thereby detecting the bending angle of the metal sheet.
In a widely known detection mechanism of the non-contact type, by the use of a plurality of range sensors, the distance from each sensor to a bent metal sheet is detected and the difference between the respective distances is obtained, thereby detecting the bending angle of the metal sheet. Examples of the above detection mechanism are disclosed in Japanese Patent Publication Laid-Open No. 63-49327 (1988), No. 64-2723 (1989) and No. 1-271013 (1989). As the range sensors, the first mechanism employs eddy-current sensors, the second employs electrostatic capacitance sensors, and the third employs optical sensors.
Such a bending angle detection mechanism of the contact type as disclosed in Japanese Patent Publication Laid-Open 1-273618 (1989) requires that the bent legs of a metal sheet should be comparatively long in order to ensure high measurement accuracy, and it is therefore difficult to apply this mechanism to a metal sheet with short legs. It has other problems in that the long use of the mechanism causes the probe to wear and to be deformed because of the long contact with the metal sheet, resulting in the deterioration of the measurement accuracy, and the space required for measurement is restricted by the lower bender. Furthermore, in the case of a thin metal sheet to be bent, the metal sheet is prone to be deflected being in contact with the probe. This also makes it difficult to ensure accuracy in the detection of the bending angle.
In the bending angle detection mechanisms of the non-contact type wherein eddy-current sensors are employed as taught by Japanese Patent Publication Laid-Open No. 63-49327(1988), or electrostatic capacitance sensors are employed as taught by Japanese Patent Publication Laid-Open No. 64-2723(1989), the output varies according to the material of a metal sheet to be bent. and therefore measuring conditions have to be altered whenever different materials are used. In the mechanism of Japanese Patent Publication Laid-Open No. 1-271013(1989) wherein optical sensors are employed, illuminated light is diffused in some surface conditions of a metal sheet. This causes large measurement errors and the deterioration of the measurement accuracy.
One of the present invention is therefore to overcome the above problems by providing a metal sheet bending machine for bending a metal sheet to a desired angle, which is capable of detecting a bending angle with high accuracy during the bending process irrespective of the length of the bent legs of the metal sheet, even if metal sheets made of different materials are used.